1. Field of the Invention
The present invention relates to a control unit for a motor-assisted bicycle, and more particularly to a control unit for driving a motor-assisted bicycle with the same running sensation as that obtained by a bicycle without a motor-assisted unit (hereinafter, referred to as “ordinary bicycle”).
2. Description of the Background Art
A motor-assisted bicycle of a type including a manually powered drive system for transmitting a force applied to pedals by manual power, e.g. a rear wheel is powered by a driving force from an operator's legs and a motor drive system for adding an assist power to the manually-powered drive system in accordance with the driving force exerted by the operator's legs. This type of motor-assisted bicycle is configured to assist manual power with a motor output in accordance with both a leg-power and a pedal rotational speed. As the leg-power increases, the motor output increases to reduce the necessary manual power and the operator's exerted energy. Accordingly, the motor output is proportional to the leg (manual) power.
A motor output for a motor-assisted bicycle has also been considered in the background art that attempts to compensate for the weight of the bicycle and to reduce power consumption, e.g. during periods when an operator may be walking the bicycle. For example, a motor-assisted bicycle has been proposed in Japanese Patent Laid-open No. Hei 8-127386, wherein a difference in weight between the motor-assisted bicycle and an ordinary bicycle is offset/assisted by a motor to reduce the required manual power when a driver walks the bicycle.
As described above, in the motor-assisted bicycles of the background art, since a motor output is proportional to the manual power, e.g. leg-power, an assist power is provided so as to amplify a periodical change in leg-power occurring in accordance with the rotation of the bicycle pedals. Accordingly, although man-power can be reduced, the vehicle speed tends vary periodically.
The above-described peripheral variation in vehicle speed does not occur where a difference in weight between the motor-assisted bicycle and an ordinary bicycle is canceled by an assisted power irrespective of the manual power. However, the motor-assisted bicycles of this type are intended to provide an assist power irrespective of the manual power only at the time when an operator walks the vehicle. Therefore, the motor-assisted bicycles of the background art do not provide an assist power irrespective of the manual power during operation of the vehicle.
Further, the motor-assisted bicycles of the background art further require another assist control means such an inclination sensor in situations where the bicycle is operated on inclined surfaces such as steep roads. The latter-described motor-assisted bicycle of the background art may often fail to suitably generate an assist power, since an assist power is corrected only on the basis of the degree of inclination of a road surface on which a driver walks the vehicle.